The present invention relates to a method for the manufacture of thermoplastic tubes and particularly to the manufacture of curved thermoplastic tubes which exhibit a consistent wall gauge and circular cross-section.
Typically, the manufacture of fuel filler tubes, fuel line tubes, radiator coolant tubes, power steering tubes, etc. used in the automotive industry, for instance, involves the steps of preparing a length of uncured, flexible tube consisting of an inner tube, an outer cover, and a reinforcement positioned therebetween, and then loading the green preform onto a rigid mandrel which may be made of metal, plastic or rubber. The mandrel has a general shape which corresponds to the desired tube configuration. The length of tube mounted on the mandrel is then cured in an open steam autoclave and then removed from the mandrel as a finished product. While this rigid mandrel process has proved largely satisfactory, three-dimensional serpentine configurations having extremely acute bends, are virtually impossible to build on rigid, curved mandrels. Not only is it difficult to load and unload the tube element with these prior methods, but also problems with respect to hot tear and creasing at the inside surface of the bends arise which are detrimental to performance characteristics, such as fluid flow, and eventually lead to premature failure of the tube in operation. Various methods have been proposed to address these concerns.
The use of granular material such as sand as a flexible mandrel material in the production of curved tubular articles, is disclosed in U.S. Pat. No. 1,877,628 to Replogle. However, the use of sand as a mandrel core in the preparation of curved tube is not satisfactory inasmuch as it produces a tough, stippled or porous interior surface upon curing and the sand particles lodge within such pores where they are virtually impossible to totally remove by flushing after cure. The occurrence of even residual amounts of such particulate material is highly undesirable in, for example, fuel filler tubes where the particles may foul the fuel pump or other intricate orifices in the fuel line system of an automobile.
U.S. Pat. No. 3,753,635 to Barnett describes an apparatus for precision bending of thermoplastic conduit comprising a rotatable means having a multiplicity of locations and a plurality of stations, each station having a pressurizing means for establishing a pressure differential across the walls of a segment of a conduit to be bent, the higher pressure being on the interior of the conduit, a clamping means for each end of the segment of conduit, a jig means having an interior wall for effecting an elbow having a desired finish, an automatic bending means connected with a movable portion of the clamping means and a stop to limit the degree of bending. The rotary device moves through a loading location, a heating location, a heating and bending location and a cooling location when the pipe is cooled by externally applying cool air to the assembly and then subjecting the pipe to a cooling bath or spray of cool water.
U.S. Pat. No. 3,021,871 to Rodgers describes an extruded spiral shaped length of tubing, preferably of nylon, wrapped around a mandrel of a size to determine the desired internal diameter of a coil of tubing. The tubing is wrapped at room temperature and then has live steam circulated through the tubing. Cold water is then passed through the tubing to fix the thermoplastic material in the desired coil shape.
U.S. Pat. No. 3,826,288 to Cooper et al. Teaches a thermoplastic article of preformed shape by wrapping a length of tubing or tube stock such as nylon around one mandrel then in the opposite direction around another mandrel spaced from the first in a figure eight configuration. The fixture containing the mandrels and the tube is then suitably heated until the tubing assumes the shape of the mandrels and is then cooled to retain that shape. Heated fluid may be circulated through the tube in the heating step and cool fluid may be used in the cooling step.
U.S. Pat. No. 3,992,505 to Tally describes an automotive radiator tube of elastomeric material by filling the interior of an uncured length of elastomeric tubing with a flowable incompressible material, capping the ends of the tubing and training the thus filled tubular element about discrete unconnected support means disposed in a configuration to represent the final shape of the article and vulcanizing tube. The material in the tube may be a water soluble salt or water. This is a thermosetting process and cooling is not necessary as the vulcanization fixes the final shape.
U.S. Pat. No. 4,218,420 to Jacob et al. Describes a method of curing a tubular tube in a spiral groove cut in mating halves of press plates. The unshaped tube is disposed in one spiral groove of the mold halves, the mold closed, and a heating step applied while the tube is confined to the spiral groove. The finished tube has a precision defined exterior. A hot fluid may be passed through the tube during the heating step.
In U.S. Pat. No. 4,144,622 to Stroupe there is described a method of forming an article of tortuous shape utilizing a metallic mold of two or more separate parts having an interior cavity of the desired shape of the finished article. The article to be shaped is built upon a tube former. The green article and the tube former are placed in the cavity, the mold parts closed and secured and the tube former is inflated to approximately 50 psi gauge. The article is then heat-cured in the mold or may be removed from the mold and post treated. These articles are made of an overlapped sheet element, a surrounding rib-knit fabric and congealing viscous resin.
In U.S. Pat. No. 3,859,408 to Voss et al., the patentees advance a tubular carcass through a generally tubular cross-sectional mold, seal both ends, one with a floating seal and pinch rolls, the other with a plug, internally pressurize the tube through the plug end, setting the carcass in the mold portion with externally heating coils, relieving the pressure, advancing a new section of carcass through the mold, severing the vulcanized portion from the green portion, sealing the open end and repeating the process. The pressurizing means preferably also comprises a heating means.
Canadian Pat. No. 1,042,642 to Barbier et al. Discloses a congealable fluid introduced into a green tube in a fluid form. The congealable fluid is converted to a solid form to complete the tube construction on a solid mandrel. The congealed fluid is then reconverted to fluid form during the vulcanization process.
Tubes manufactured to have a three-dimensional serpentine configuration typically are plagued with a constrictive diameter at the point of the bend, particularly when the bend is at an acute angle. The constrictive diameter is due to wrinkling of the smaller inner circumference of the bend which prevents the inner diameter of the tube from being circular. Therefore, such tubular structures exhibit circular diameters at locations where the tube is straight, but in areas containing bends, especially acute bends, they exhibit oval shapes. While the total areas may be the same or very similar, the particular shape of the tube at the bends drastically affects the performance of the tube. Therefore, a need exists in the industry for a method of manufacturing thermoplastic tubes having various three-dimensional serpentine configurations including even extremely acute bends which exhibit a consistent wall gauge and circular cross-section uniformly throughout their entire length, without the difficulties encountered in the previous prior art.